Cleanup: Pass std::string as const reference if possible

google_apis/gaia/oauth2_token_service_test_util.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "google_apis/gaia/oauth2_token_service_test_util.h"
 
 #include "base/strings/stringprintf.h"
 
 namespace {
 const char kValidTokenResponse[] =
     "{"
     "  \"access_token\": \"%s\","
     "  \"expires_in\": %d,"
     "  \"token_type\": \"Bearer\""
     "}";
 }
 
-std::string GetValidTokenResponse(std::string token, int expiration) {
+std::string GetValidTokenResponse(const std::string& token, int expiration) {
   return base::StringPrintf(kValidTokenResponse, token.c_str(), expiration);
 }
 
 TestingOAuth2TokenServiceConsumer::TestingOAuth2TokenServiceConsumer()
     : OAuth2TokenService::Consumer("test"),
       number_of_successful_tokens_(0),
       last_error_(GoogleServiceAuthError::AuthErrorNone()),
       number_of_errors_(0) {
 }
 
 TestingOAuth2TokenServiceConsumer::~TestingOAuth2TokenServiceConsumer() {
 }
 
 void TestingOAuth2TokenServiceConsumer::OnGetTokenSuccess(
     const OAuth2TokenService::Request* request,
     const std::string& token,
     const base::Time& expiration_date) {
   last_token_ = token;
   ++number_of_successful_tokens_;
 }
 
 void TestingOAuth2TokenServiceConsumer::OnGetTokenFailure(
     const OAuth2TokenService::Request* request,
     const GoogleServiceAuthError& error) {
   last_error_ = error;
   ++number_of_errors_;
 }